1. Field of the Invention
The present invention relates to a method for crystallizing optically active tryptophan.
2. Discussion of the Background
In the crystallization of L-tryptophan (hereafter simply referred to as L-Trp), it is known that pigments and other materials can be removed from the crystallization medium to crystallize tryptophan in good selectivity. L-Trp is isolated by adding an organic solvent such as isopropyl alcohol, or the like to an aqueous solution of L-Trp (Japanese Patent Application Laid-Open Nos. 59-39875 (scaly crystals), 60-30694 (scaly crystals), 61-12607 (scaly crystals), 63-177796 (scaly crystals)). In these cases, it is necessary to recover the organic solvent so that these processes are not advantageous from an economic viewpoint.
On the other hand, where L-Trp is crystallized from an aqueous solution by concentration without using an organic solvent, the precipitated crystals obtained are generally fine scaly crystals. Upon isolation using a centrifuge, these crystals form layers which result in poor separation of the mother liquor and a long period of time is required for isolation of the Trp crystals. The thus obtained crystals are dried only with difficulty which means a long period of time is also required for drying. In addition, when crystallization occurs through concentration, vigorous foaming sometimes makes concentration impossible (Japanese Patent Application Laid-Open No. 60-237054 (scaly crystals)). One method of improving crystallization has been reported in which a water-soluble cellulose derivative, a water-soluble polyvinyl compound or the like is added to an aqueous solution of L-Trp. At the same time, foaming is prevented and crystallization is effected by concentration (Japanese Patent Application Laid-Open No. 60-237054 (spherical crystals)). However, when a defoaming agent or a crystallization catalyst is added, there is a danger that these compounds might be brought into the final product.
As an alternative to crystallization by concentration, crystallization can be advantageously achieved through neutralization by adding an alkali or an acid to an acidic or aqueous alkaline solution of L-Trp. One known embodiment of the method involves adding an alkali or a acid to the total amount of an acidic or alkaline aqueous solution of L-Trp until its pH becomes neutral and mixing the solution by stirring. In this case, scaly micro-crystals are generally precipitated which means that solid-liquid separation and drying of the product are unsatisfactory.
As stated above, it is difficult to crystallize and isolate L-Trp from an aqueous solution containing impurities. In most cases, impurities such as pigments, and the like are generally removed by using a synthetic resin or the like as an adsorbent prior to crystallization. One example to this method involves treating the aqueous solution with a weakly basic anionic exchange resin or an amphoteric ion exchange resin and then performing crystallization by concentration. (Japanese Patent Application Laid-Open No. 1-112991 (scaly crystals)). Even with this method, when crystallizing L-Trp, foaming is noted upon concentration, although the foaming is less vigorous because of the effect achieved by the treatment of the medium with resin to remove impurities. It is thus difficult to concentrate a slurry for crystallization of large quantities of L-Trp, without adding a defoaming agent to the system.
On the other hand, crystallization techniques other than crystallization by the addition of an organic solvent and crystallization by concentration are known, one of which is a method of forming layered flocculated crystals in which good solid-liquid separation is realized. The method comprises gradually adding a saturated solution of L-Trp at a high temperature to a slurry of L-Trp at a low temperature (Japanese Patent Application Laid-Open No. 62-265254). However, the dependency of L-Trp solubility on temperature is not as significant as for other amino acids (solubility of L-Trp in 100 g of water is 0.82 g at 0.degree. C., 1.14 g at 25.degree. C., 18 g at 40.degree. C., 2.4 g at 60.degree. C. and 3.4 g at 80.degree. C.). Where crystallization upon cooling is performed, the rate of crystallization decreases. In order to enhance the rate of crystallization, it is necessary to elevate the temperature of the saturated L-Trp solution to about 80.degree. C. In this case, however, a problem is encountered which is that decomposition of L-Trp is accelerated. A need therefore continues to exist for an improved method of crystallizing optically active tryptophan.